The sulfur removal from light oil is extremely important in the petroleum-processing industry. Several processes have been proposed in the past to deal with the problem of removing these compounds from light oil. The most important and common industrial process is that of treating the fuel under high temperatures and high pressures with hydrogen. This process is called hydrodesulfurization (HDS) and has received extensive attention since its discovery in 1930's.
The sulfur compounds contained in petroleum fuels include aliphatic molecules such as sulfides, disulfides, and mercaptans as well as aromatic molecules such as thiophene, benzothiophene, dibenzothiophene, and alkyl derivatives such as 4,6-dimethyl-dibenzothiophene. Where, the conventional HDS technology can desulfurize aliphatic and cyclic sulfur-containing organic compounds on an industrial scale, as in most refineries in the world. Meanwhile, the aromatic dibenzothiophene (DBT) and especially 4,6-alkyl-substituted DBTs are difficult to convert to H2S due to the sterically hindered nature of these compounds on the catalyst surface (Shiraishi Y. et al. 2002). Additionally, from environmental and economic viewpoints, it is desirable to develop a more energy-efficient desulfurization process for production of virtually sulfur-free fuel due to the requirements of high temperature, high H2 pressure and hence a larger reactor as well as an active catalyst.
Oxidative desulfurization (ODS) has been considered as a further new promising technology for deep desulfurization of light oil because it can be carried out under mild conditions, such as relatively low temperature, pressure and cost of operation when it is compared with HDS (Breysse et al., 2003). This desulfurization process includes two stages: (i) oxidation in a first step; and (ii) liquid extraction at the end. It is evident that the greatest advantages of the ODS process are low reaction temperature and pressure, and that expensive hydrogen is not used in the process. Another feature of ODS is that the refractory S-containing compounds in ODS are easily converted by oxidation.
Sulfur-containing compounds are oxidized using a selective oxidant to form compounds that can be preferentially extracted from light oil due to their increased relative polarity. Such oxidants include peroxy organic acids, hydroperoxides, nitrogen oxides, peroxy salts and ozone, etc. and such oxidants can donate oxygen atoms to the sulfur in mercaptans (thiols), sulfides, disulfides and thiophenes to form sulfoxides or sulfones (Campos-Martin, et al., 2010). Several oxidation systems have been studied, such as H2O2/heteropolyanion (phase transfer catalyst) (Wan and Yen, 2007), H2O2/formic acid system (Hao, et al., 2005).
Superoxides, for instance, potassium superoxide, have been demonstrated as alternative oxidants for the ODS process. For model compounds of benzothiophene, dibenzothiophene, and a number of selected diesel oil samples, sulfur removal greater than 90% and as high as 99% was accomplished (Chan et al., 2008). The results for using this solid potassium superoxide are comparable to or better than the results with liquid hydrogen peroxide for the ultrasound-assisted oxidative desulfurization (UAOD) or ODS process. Super oxide anion, O2−. is a free radical having one unpaired electron. Many types of superoxides are stable at dry ambient conditions even in high purity. Upon contact with water, it dissociates forming O2 and H2O2. Therefore, these materials can provide high active oxygen ratio as in the case of potassium superoxide which has an active oxygen ratio of 45 wt. % (Chan, 2010). In addition, potassium permanganate and sodium superoxide are used efficiently under the effect of UV-irradiation in ultrasound assisted system for inducing oxidative desulfurization of some model sulfur compounds. KMNO4 and NaO2 induced removal of sulfur compounds (BT and DBT) with a maximum of >98%. When applying potassium superoxide to marine gas oil, jet propellant 8 and sour diesel in the presence of some of the ionic liquids and under the effect of temperature, the desulfurization brought a maximum of about 98%, 99% and 95%, respectively.
Using some special additives like ethylene diamine tetraacetic acid (EDTA), magnesium silicate and sodium silicate could enhance the desulfurization process of cooker gas oil (CGO) under the effect of hydrogen peroxide/formic acid system. These additives were selected to catalyze hydrogen peroxide decomposition, thus improving oxidation efficiency and extraction process more effectively. In case of using EDTA with H2O2/formic acid system, the desulfurization of CGO reached 90% (Hao, et al., 2005). In addition, metal oxides are found to be more reactive towards the compounds of sulfur, especially thioles compounds. Several of metal oxides like MnO2, PbO2, Al2O3, MgO2, ZnO2 and silica have been investigated in desulfurization of Jhal Magsi crude oil and its distillation fractions (kerosene and diesel). The results indicate that PbO2 and MnO2 caused a more significant effect of sulfur depletion in all three samples (Jhal Magsi crude oil, kerosene and diesel) than in the case of other oxides. According to these studies, lead oxide and manganese oxide achieved a maximum desulfurization in crude oil of about 55.35% and 45.18%, respectively in the case of reaction time 1 hour. In the case of kerosene and diesel, lead oxide achieved a maximum sulfur removal of about 49.05% and 54.54%, respectively, during the reaction time of 1 hour. Increasing the reaction time between magnesium oxide and Jhal Magsi crude oils and its distillate fractions (kerosene and diesel) enhances the desulfurization process (Shakirullah, et al., 2009).